This application incorporates by reference the disclosures of co-pending patent applications Ser. No. 09/651,688 by Masashi Takayanagi entitled xe2x80x9cFour-Cycle Outboard Motorxe2x80x9d and Ser. No. 09/651,452 by Jun Itoh entitled xe2x80x9cEngine Holder Structure for Four-Cycle Outboard Motorxe2x80x9d, each of which is filed concurrently with the present application and is assigned to the assignee of the present application.
1. Field of the Invention
The present invention relates to a four-cycle outboard motor.
2. Description of the Related Art
The engine used in an outboard motor is generally a vertical-type engine, in which a crankshaft is arranged vertically. A drive shaft for transmitting the rotational force of the crankshaft to a propeller is arranged coaxially with the crankshaft such that the drive shaft is directly connected to the crankshaft.
Further, the engine layout is typically such a configuration that, with the vertical crankshaft crossing the center line along the fore-aft direction of the outboard motor, a crankcase is arranged on the front side (may be referred to xe2x80x9cfrontxe2x80x9d) of the outboard motor and that a cylinder head is arranged on the rear side (may be referred to xe2x80x9crearxe2x80x9d), respectively.
Furthermore, in the case of a four-cycle engine, a pulley or sprocket is disposed on the crankshaft so that a valve camshaft is driven by way of a belt or chain. The pulley or sprocket disposed on the crankshaft is generally arranged between a bearing at the uppermost portion of the crankshaft and a flywheel disposed at the top end of the crankshaft.
However, when the crankcase and the cylinder head are arranged on the front side and on the rear side of the outboard motor, respectively with the crankshaft crossing the center line along the fore-aft direction of the outboard motor, the center of gravity is located toward the rear of the outboard motor. This biased position of the center of gravity is especially toward the rear in the case of a four-cycle engine since a valve mechanism is provided in the cylinder head.
An outboard motor is typically mounted to a hull by way of a mounting device so that it is constituted to be able to steer to the left or the right by turning itself around a pilot shaft (outboard motor""s axis of rotation). However, the control stability becomes less when the center of gravity of an outboard motor recedes farther from the pilot shaft.
Further, since the mounting device is attached to the outboard motor by way of a mounting unit, vibration may be increased if the center of gravity of the outboard motor recedes farther from the mounting unit.
Meanwhile, in the case of a four-cycle engine, because the rotational speed ratio of the crankshaft and camshaft is 2 to 1, the ratio of the numbers of teeth (circumference) of the sprocket (pulley) disposed on the crankshaft and the sprocket (pulley) disposed on the camshaft is 1 to 2. Thus, because the diameter of the sprocket (pulley) disposed on the camshaft is univocally determined by the diameter of the sprocket (pulley) disposed on the crankshaft, there is a limit to decreasing the diameter of the sprocket (pulley) disposed on the camshaft. Due to this limitation, it is not easy to reduce the size of the valve mechanism of an engine having two camshafts.
Furthermore, when a camshaft drive sprocket (pulley) is arranged between the uppermost bearing of the crankshaft and the flywheel, the engine is apt to vibrate due to the large inertial mass of the flywheel disposed at the top end of the crankshaft.
The present invention was made in view of the above situation, and it is an object of the present invention to provide a four-cycle outboard motor where vibration is reduced, where control stability is improved, and where the overall outboard motor is more compact, by moving the location of the center of gravity toward the front.
Another object of the present invention is to provide a four-cycle outboard motor in which a valve apparatus is more compact.
The present invention satisfies the above-described needs by providing a four-cycle outboard motor having a crankshaft arranged substantially vertically within an engine so that the rotational force of this crankshaft is transmitted to a propulsion unit via a drive shaft, the four-cycle outboard motor being characterized in that the shaft centers of the above-mentioned crankshaft and the above-mentioned drive shaft are arranged, in the plan view, either on an axis of a cylinder constituting the above-mentioned engine or on the center line of the engine, which extends in the fore-aft direction of the above-mentioned outboard motor and passes through the shaft center of the above-mentioned crankshaft, and in that the shaft center of the above-mentioned drive shaft is arranged so as to be offset toward the rear of the above-mentioned outboard motor from the shaft center of the above-mentioned crankshaft.
Further, it is another object of the present invention to provide a four-cycle outboard motor having a crankshaft arranged substantially vertically within an engine so that the rotational force of this crankshaft is transmitted to a propulsion unit via a drive shaft, of which the shaft center is arranged so that it is disposed offset from the shaft center of the above-mentioned crankshaft toward the rear of the above-mentioned outboard motor, the four-cycle outboard motor being characterized in that a rotational force transmitting member such as sprocket and pulley for driving the valve camshaft is arranged lower than the axis of the bottommost cylinder constituting the above-mentioned engine, and in that the above-mentioned rotational force transmitting member is arranged coaxially with the above-mentioned drive shaft.
Furthermore, it is another object of the present invention to provide a the four-cycle outboard motor having the shaft centers of the above-mentioned crankshaft and the above-mentioned drive shaft arranged, in the plan view, either on an axis of a cylinder constituting the above-mentioned engine or on the center line of the engine, which extends in the fore-aft direction of the above-mentioned outboard motor and passes through the shaft center of the above-mentioned crankshaft.
And furthermore, it is another object of the present invention to provide a four-cycle outboard motor having a first rotational force transmitting member (valve camshaft driving sprocket or pulley), which is arranged coaxially with the above-mentioned drive shaft, and a second rotational force transmitting member (valve camshaft driving sprocket or pulley), which is disposed on the above-mentioned valve camshaft, with the speed conversion ratio of the first rotational force transmitting member and the second rotational force transmitting member set less than 2 to 1 (or 2/1). The speed conversion ratio, for example, may be defined by a ratio (n1/n2) of the number (n1) of teeth of a valve camshaft driving sprocket arranged on the valve camshaft and the number (n2) of teeth of a valve camshaft driving sprocket arranged coaxially with the drive shaft if sprockets are employed.
Further, it is another object of the present invention to provide a four-cycle outboard motor having at least two valve camshafts: one for intake and one for exhaust.
Still further, it is another object of the present invention to provide a four-cycle outboard motor having a crankshaft and a drive shaft, which are connected by way of connecting means, the outboard motor further comprising a crank gear disposed on the bottom end of the crankshaft and a driven gear, with which the crank gear engages, wherein the driven gear is integrally formed into the connecting means and wherein a first rotational force transmitting member (valve camshaft driving sprocket or pulley) is integrally formed with the connecting means.
Further, it is another object of the present invention to provide the above-mentioned four-cycle outboard motor having the first rotational force transmitting member (valve camshaft driving sprocket or pulley) for driving the valve camshaft arranged below the driven gear.
Furthermore, it is another object of the present invention to provide the above-mentioned four-cycle outboard motor having the first rotational force transmitting member (valve camshaft driving sprocket or pulley) for driving the valve camshaft arranged above the driven gear.
And then, it is another object of the present invention to provide the above-mentioned the four-cycle outboard motor wherein the crank gear is pressed onto the bottom end of the crankshaft.